Urko has been relieved of his duties for three months because of his recent negligence involving his leadership and the failed attempts at capturing Blue Eyes and the rest of the humans. Col. Rotok will replace him temporarily and Urko insists his men obey Rotok to the letter. In their lab, Zira admits to Cornelius that she actually feels sorry for Urko, but he reminds her what Urko would do to the humans if he returns to power. With Urko out of the picture, they decide to risk a trip to the high mountain apes to retrieve an ancient book proving that man once ruled the planet. Perhaps by showing the senate the evidence, laws will be passed granting man some kind of protection . While they set off to find Jeff and Bill, Urko realizes that another attack on the humans cannot wait an additional three months. By then, their defenses will be even stronger. He convinces Rotok (who remains loyal to Urko) that if he will lead a surprise attack on the still-unprotected west flank of New Valley, they would all be heroes. After Cornelius tells Bill about the book, they decide that the mountain peaks would be too treacherous for the P-40 and to walk would take weeks. Instead, they decide to repair and fly the hot air balloon used to fly to Kigor's mountain once before. Unknown to them, the army is approaching New Valley and a scout patrol is sent ahead to verify the humans' position. Bill and Cornelius decide to make the balloon trip together as a gesture to the High Llama that apes and humans are together in this decision. They are spotted on takeoff by the scouts, whose clumsy driving awakens the monster bird from the lake where it landed following Judy's dog fight with it. The monster attacks the balloon, so Bill steers for the clouds to hide. The scouts report the balloon sighting to Col. Rotok. Everyone wants to delay the attack until the nature of the balloon is known, but Rotok orders the attack to proceed. When Jeff, Judy, and Zira realize that the army is closing in, Jeff creates a diversion to allow Judy to get to the plane. The army runs frightened from the old aircraft. The balloon is being battered by a storm. Finally, they spot Kigor's temple and land nearby. The High Llama takes them to the foot of Kigor's giant frozen image to dig up the book they came for, "A Day at the Zoo." While returning to the temple in an ancient ski lift car, the monster bird appears and attacks again. Suddenly, the ice over Kigor's huge frame crumbles and the giant mountain ape fights off the bird. Bill and Cornelius then mend Kigor's wounds before flying home, hoping the book will finally bring unity to the planet.